Over the last century, mankind has developed a growing understanding of the nature of light. This growing understanding has led to an increasing ability to harness and control light, which has in turn led to improvements in a wide variety of different technologies. For instance, the ability to control photons has led to improvements in communications, such as through the development of fiber optics; improvements in opto-electronics, such as through the development of photo-voltaic cells; as well as the development of near-field optics, a field of study dedicated to the utilization of near-field light, which is the light created around the periphery of an object emitting or being illuminated by light. The study of near-field light has brought about the development of and continuing improvements to many optical devices including many different types of imaging devices as well as optical scanners, filters, switches, modulators, and the like.
For many years, the low transmittivity of light waves through extremely small diameter holes, smaller than the wavelength of the incident light, was a limiting factor to the further development of near-field optical devices. Specifically, the transmission of light of a wavelength λ through an aperture of diameter d, where d<λ, was found to be proportional to (d/λ)4. (See for example published U.S. patent application 2003/0185135 to Fujikata, et al.) Recently, however, it has been discovered that the level of optical transmission through such holes can be strongly enhanced through the formation and utilization of a surface plasmon polariton band structure on the surface of the metal film containing the apertures. Specifically, the formation on a metal film of a periodic array of sub-light wavelength surface structural elements, e.g., holes, dimples, or concentric rings, can give rise to the formation of a well-defined series of surface plasmon polaritons on the film. Surface plasmon polaritons (also referred to throughout this disclosure as simply plasmons or SPP) exist when light couples with surface plasmons, which are collective electronic excitations at the interface of a metal (or metallic material) with an adjacent dielectric material. The SPP can be resonantly excited by the impingement of incident light of a particular wavelength. Specifically, the resonant coupling between the incident light and the plasmons exist at wavelengths that have been shown to be dependent upon the geometry of the periodic array formed on the material, the angle of incidence of the incident light, as well as the refractive index of the dielectric material adjacent to the metal film. The resonantly excited plasmons can propagate through the apertures in the metal film to the other side and, since their wave nature represents acceleration in electronic charge, they can subsequently reradiate the impinged light.
One of the net results of the resonant coupling of plasmons with incident light can be induced transparencies in nominally opaque materials that support a plasmon resonance (i.e., permittivity, ε, being less than zero) when light of the correct wavelength strikes the materials. In addition, because the photon/plasmon interactions are so strong, the effects of the resonant coupling effect can be highly efficient. For instance, these structures can exhibit greater than 100% transmission when compared to the transmission expected from the total area defined by the holes.
Research is continuing in an effort to expand the application of these devices. For instance, Kim, et al. (U.S. Pat. No. 6,040,936), which is incorporated herein by reference, disclose an optical transmission modulation apparatus including a metal film having a periodic array of sub-light wavelength-diameter holes and a supporting layer including a material displaying a selectively variable refractive index. The material displaying a selectively variable refractive index can be, for example, a liquid crystal material, a ferro-electric liquid crystal, a semiconductor layer, or a polymer electro-optic film, i.e., materials in which the refractive index can be controlled through application of an electric field to the material.
Though such advances represent great improvement in the art, there remains room for variation and further improvement in the field.